Pyridine and picolines (where a methyl group, attached to the carbon ring, can be present at three different regio positions, with respect to ring nitrogen, such as 2-methyl pyridine or α-picoline, 3-methyl pyridine or β-picoline and 4-methyl pyridine or γ-picoline) are important intermediate compounds in the manufacture of agricultural chemicals (like herbicides and pesticides) and pharmaceuticals. They are also useful as specific solvent in different industries like textile, polymer & pharmaceuticals.
Although, the pyridine and picolines can be obtained as by-products in coal tar industry, the preferred method for obtaining pyridine and picolines is by chemical synthesis, mainly because of small amount of pyridine and picolines present in coal tar. Chemical method for the synthesis of these pyridines and picolines is based on a catalytic process where carbonyl compounds such as an aldehyde represented by formaldehyde, acetaldehyde, propionaldehyde and/or a ketone such as acetone, propionone and the like are reacted with ammonia in gas phase over a bed of solid catalyst such as amorphous silica-alumina (see for example U.S. Pat. No. 2,807,618) and crystalline aluminosilicates, which are commonly known as zeolites (see for example U.S. Pat. No. 5,994,550).
Alumino-silicate zeolites and their metallosilicate analogues are crystalline, microporous silica based materials having different framework structures. When a trivalent metal ion like B3+, Al3+, Fe3+, Ga3+, As3+ etc. are incorporated in a crystalline silica network, a net negative charge is generated on the framework. This net negative charge is balanced by another extra framework charge compensating, ion-exchangeable cation. When proton is present as charge compensating cation then there is no net negative charge generated and the zeolite framework remains neutral without ion-exchange property. Although, such zeolite having certain tetravalent metal ions, particularly transition metal ions other than Si, with neutral framework do not exhibit proton donating Bronsted acidity, it is likely depending upon the chemical nature of the incorporated metal ion other than Si that these zeolites exhibit remarkable redox (see for example Kumar et al., SYNLETT, Year 1995 pages 289-298) and Lewis acid characteristics (see for example M. Shashidharan and R. Kumar, ‘Titanium-silicate molecular sieves (TS-1 & TS-2) catalyzed Michael reaction of silylenolethers with α,β unsaturated carbonyl compounds’, Catalysis Letters, volume 38, year 1996, pages 251-254, and M. Shashidharan et al. ‘Titanium-silicate molecular sieve, TS-1, catalyzed C—C bond formation in Mukaiyama type aldol reactions’ Chemical Communications, year 1996, pages 129-130).
Aiming to improve the overall yield of desired pyridine and picolines, various zeolite catalysts where frame work aluminum is replaced either fully or partially, by one or more cation(s) selected from divalent cations like Co2+ (see U.S. Pat. No. 6,281,362) trivalent metal ions like Fe3+ and/or Ga3+ (see U.S. Pat. No. 4,810,794) or tetravalent metal ions like Ti4+ (see U.S. Pat. No. 6,281,362), in the zeolite tetrahedral framework, commonly known as metallo-silicate analogues of their corresponding alumino-silicate zeolites, are also used as catalyst. For example, in U.S. Pat. No. 4,810,794 Shimizu et al and in U.S. Pat. No. 5,952,258 Saitoh et al have claimed the use of a zeolite having Si and B, Al, Fe, and/or Ga as zeolite constituent element, where an atomic ratio of Si to B, Al, Fe and/or Ga of 12 to 1000, as catalyst for producing pyridine and picolines. Among a large number of zeolites with different structure or topology used as catalyst, zeolite with MFI type topology, commonly known as ZSM-5, provides superior performance.
However, the main drawback of these catalysts was relatively low yields of desired pyridine or picolines and quick deactivation of the catalyst. In order to improve the yield of the main products (pyridine or picolines) and catalysts life, other metal ions selected from group I to XVII are deposited on the zeolite catalyst via post synthesis modification (see for example U.S. Pat. Nos. 4,810,794; 4,866,179, 5,994,550 and 6,281,362). Recently, in U.S. Pat. No. 6,281,362 Iwamoto teaches that when a catalyst comprising Ti and/or Co along with Silica as zeolite constituent, commonly known as titanium-silicate and/or cobalt silicate having MFI or MEL (commonly known as pentasil structure) zeolite framework and preferably loaded with Pb, T1 etc., is contacted with an aldehyde or ketone and ammonia in gas phase in the temperature range of 300-700° C., the overall yield of picolines is improved substantially compared to when Al, Fe and/or Ga was used as zeolite constituent along with Silica. From above mentioned prior art methods for the production of pyridine and picolines, it can be construed that not only zeolite structure, a physical factor, but also the different metal constituent present both in the zeolite framework (as zeolite constituent) and non-framework positions (loaded by conventional post synthesis treatment), known as chemical factors, significantly influence the activity, selectivity and overall productivity of the catalyst.
However, in the above mentioned prior art method for producing pyridines and picolines using solid zeolite catalyst particularly titanium-silicate catalyst do not provide the effect of the passivation of the external surface of the catalyst and the crystal size. These are very important factors, which can affect significantly both the catalytic activity and the selectivity towards desired products, because in addition to the chemical characteristics of zeolites like the chemical nature of metal ions present in the zeolite structure, physical or morphological nature of the crystals can also significantly influence the activity, selectivity and productivity of a zeolite catalyst in a given reaction. For example, shape and size of the zeolite crystals, hereinafter denoted as crystallites, having same ZSM-5 frame work structure and same chemical constituents significantly influence the activity, selectivity and productivity in shape selective reactions like xylene isomerization (see for example: Influence of crystal size of ZSM-5 on activity and selectivity in xylene isomerization by Ratnasamy et al in journal Zeolites volume 5, pages 98-100 published in March 1986). In general, smaller crystallites are more active and less shape selective. However, there exists an optimum value of the crystallite size to achieve the maximum possible productivity of the catalyst in a given catalytic application. Further, the non-selective contribution of external surface of the crystallites also depends on the morphology of the crystallites. The selective passivation of the external surface of the crystallites by conventional post-synthesis methods like treating the zeolite crystallites with silicon tetrachloride or silicon tetraethoxide (see for example W. W. Kaeding and S. A. Butter, U.S. Pat. No. 3,911,041 in 1975 and M. Niwa et al. in Journal of Chemical Society Faraday—I, volume 81, page 2757 and year 1985) may also lead to improved selectivity of the desired products.